The present invention relates generally to systems for removing interference distortion such as multipath distortion contained in signals obtained by demodulating frequency-modulated signals. More particularly, the invention relates to a system wherein a distortion component closely approximating an interference distortion contained in a signal obtained by demodulating a frequency-modulated signal is formed and used to cancel and remove the interference distortion.
In general, when a plurality of frequency-modulated signals (hereinafter referred to as FM signals) interfere with each other, interference distortion is produced. This phenomenon occurs in cases such as that, for example, wherein FM signals which have passed through a plurality of propagation paths (multipaths) interfere with each other, and this interference distortion is called multipath distortion. For example, in the case where an FM broadcast wave is directly received at the antenna of an FM receiver and, at the same time, is received after being reflected or diffracted by objects such as buildings, the FM signals thus received after passing through multipaths in this manner interfere with each other and give rise to interference distortion in the demodulated signal.
In order to reduce the generation of multipath distortion of this character, it has been a practice in known FM receivers to use antennas of sharp directional characteristic by which only directly arriving FM signals can be readily received, while FM signals arriving after being reflected or diffracted are not easily received. However, the occurrence of multipath distortion cannot be fully reduced by merely relying on only the directional characteristic of the antenna.
Accordingly, a method wherein a signal approximating the interference distortion component is formed by an electrical circuit within the FM receiver and used to cancel the distortion component in the demodulated signal has been tried. In this known method, however, the degree of approximation of the signal caused to approximate the distortion for the purpose of cancelling the distortion component is very rough. For this reason, the signal components of the signal for distortion cancellation and the actual distortion component, strictly speaking, are considerably far apart, whereby distortion cancellation cannot be satisfactorily carried out, and, even after the distortion cancellation operation, a substantial distortion component remains.
In this connection, mutual interference of FM signals is not limited to only FM broadcasting but occurs in other instances such as the case of recording and reproducing of an FM signal on and from a recording medium and the case where there is mismatching between a cable for transmitting an FM signal and the related input and output equipments.
The present applicant has previously proposed a "System for removing interference distortions in the demodulated signal of a frequency-modulated signal" by the Patent Application Ser. No. 909,663 filed May 25, 1978 and now U.S. Pat. No. 4,166,251. As described hereinafter in detail with reference to drawings, this previously proposed system comprises a demodulator for demodulating an input FM signal which has been subjected to interference by another FM signal, an envelope detector for detecting the envelope of fluctuation of the amplitude of the input signal, an AGC circuit for controlling the level of the input signal supplied to the envelope detector, a capacitor for passing only the AC component of the output of the envelope detector, a circuit having a transfer characteristic of a hyperbolic function and converting the waveform of the AC component, an analog multiplier supplied at its x input terminal with the output of the waveform converting circuit and producing a distortion cancellation signal, a first operation circuit supplied with the output signal of the demodulator and with the distortion cancellation signal and carrying out an operation such as substantially cancelling the interference distortion of the demodulated signal with the distortion cancellation signal, a delay circuit for delaying the output signal of the first operation circuit or of the demodulator by a specific time, and a second operation circuit supplied with the output signal of the first operation circuit or of the demodulator and with the output signal of the delay circuit, thereby carrying out a signal combining operation and supplying the resulting output signal to the y input terminal of the analog multiplier. The output of the first operation circuit is a demodulated signal in which the interference distortion has been cancelled and thus removed.
In a case where a level of an unreflected signal directly received from a transmitting antenna (referred as a directly-received signal) is larger than a level of a signal received after being reflected by the building (referred as a reflected signal), a frequency demodulated signal, in which the interference distortion has been reduced or removed, is obtained by the previously proposed system. However, in a case where the level of the directly received signal is smaller than the level of the reflected signal, the previously proposed system is not able to remove the interference distortion but rather increases the distortion.
In actual practice for receiving FM signals, the ratios of magnitudes of levels of the directly-received signal and reflected signal are various according to circumferential conditions around the receiving point or location and conditions of transmitting paths of FM signals. In some above described conditions, there is a case where the level of the reflected signal is larger than that of the directly-received signal. In that case, a received and demodulated signal of larger S/N ratio is obtained by receiving and demodulating the reflected signal rather than the directly-received signal.